The present invention relates to a data transmission system employing a plurality of parallel channels, and more specifically to improvements in a timing phase control apparatus used on the receiving side.
The multiplexed parallel transmission system has been widely employed. In this transmission system, various distortions and deviations, such as phase and amplitude distortions and a sampling, timing phase deviation, are absorbed or compensated by an equalizer such as transversal filter, so that the data can be correctly demodulated. As a compensating system for the timing phase deviation, there has heretofore been employed a timing synchronization system which controls the oscillation frequency of a voltage-controlled oscillator (VCO) to optimize the sampling timing phase on the basis of a timing phase deviation data obtained by the equalizer according to the following principle. That is, the distribution of tap coefficient (weight ocefficient) values of the transversal filter expresses the impulse response of the transversal filter. The timing phase deviation makes a tap position giving a maximum coefficient value deviate from the central tap position. Therefore, in the above system, the timing phase deviation data is detected from the change in tap coefficient value, and the voltage-controlled oscillator is so controlled that the maximum coefficient value is located at the central tap position at all times. According to GOTTFRIED UNGERBOEK, for example, the deviation of the tap position giving the maximum tap coefficient is detected by comparing the sum of squared powers of coefficients of the input side and that of the output side with the central tap es a boundary, and the tap coefficients are finely adjusted so that there is no imbalance between the two sums of the squared powers. The details are disclosed in a paper "Fractional Tap-Spacing Equalizer and Consequences for Clock Recovery in Data Modems", IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM-24, NO. 8, August, 1976. pp. 856-864.
In a conventional timing synchronizing system, a few channels, whose transmission deteriorations are expected to be relatively small, are selected among a plurality of the parallel channels and used as reference channels throughout the operation of the system. The timing phase deviation data is extracted from the reference channels to control the voltage controlled oscillator. Thus, the tap coefficients of all channels are controlled always based upon thus obtained timing phase deviation data. In the shortwave transmission system, however, unexpected selective fading or radio interference may seriously deteriorate the above-mentioned predetermined channels. Therefore, if the timing synchronization is performed based upon the timing phase deviation data of low accuracy (small S/N ratio) obtained from the thus deteriorated channels, a proper equalization operation is not expected. This causes error in the demodulated data and nonsynchronism.